Method of power control and cells site selection

ABSTRACT

A method for use in a cellular communication system is disclosed which dynamically assigns a communication device to a cell site and which dynamically assigns transmit power levels to the communication device. The method is based on measurements of interference levels at cell sites and on the path gain between the communication device and the cell site.

[0001] This application is a continuation of Ser. No. 08/281956 filed onJul. 28, 1994.

TECHNICAL FIELD

[0002] The invention relates to the area of cellular communications.

BACKGROUND OF THE INVENTION

[0003] The tremendous growth experienced by wireless communicationssystems in the past few years has transformed mobile communications froma specialized service for a select few into a service available toeveryone. Only a few years ago mobile communications systems used alimited number of narrowband radio channels for transmitting andreceiving voice information in a single geographic area whose extent wasdefined by the range of the mobile transmitter. Today's wirelesssystems, and in particular present cellular-based systems, are designedto permit an increased number of users to have access to widerbandwidths (to support data communications as well as voice) over awider geographic area. Despite such advances in wireless technology,there is a constant demand for increased system capacity whilemaintaining the quality of services to users.

[0004] The cellular communications concept calls for dividing ageographic service area into a number of cells. Each cell has anassociated cell site (also called a base station) connected to thepublic telephone network. The cell site establishes a wireless link overradio channels with communication devices (hereinafter “user devices” or“devices”) operated by system users within the cell who wish to send andreceive information (e.g. text, audio, speech, video) via the publictelephone network. Note that users of the system may be mobile orstationary, and the communications of mobile users traveling from afirst cell to a second cell can be “handed-off” to the cell site in asecond cell without an interruption in communications.

[0005] The design of cellular systems and the selection of operatingparameters for the system are particularly challenging for severalreasons. First, cellular systems have a limited number of radio channelsthat may be used, and maximizing system capacity through effectiveutilization of the frequency spectrum is crucial. Second, even withlimited spectral resources, a cellular system must be reliable, andtypically cellular systems must assure each user of a quality of servicelevel, i.e. a guaranteed minimum bandwidth (in bits per second) and aguaranteed maximum bit error rate. Third, the configuration of users,i.e. the number of users and their locations, is dynamic. For example, agiven user may travel from one cell to another during a singlecommunication, or one user with a certain quality of service requirementmay terminate a communication in one cell while another user withanother quality of service requirement in another cell initiates acommunication. Dynamic user configurations thus make it difficult tooptimize certain system parameters (e.g. cell site location).

[0006] Despite these challenges, the current, first-generation cellularsystems, based on analog FM technology, have proved to be verysuccessful. In these systems, interference between the communications ofdifferent users in different cells is kept to minimal levels bypermitting each cell to use only a subset of the available radiochannels. System capacity is maintained through reuse of radio channelsin cells that are far enough apart so as to minimally interfere witheach other.

[0007] With the objective of further reducing interference andincreasing capacity, second-generation cellular systems—based on digitalradio technology and advanced networking principles—are now beingdeveloped and deployed worldwide. Because spread spectrum is a usefultechnique for facilitating communications when large numbers of userswish to communicate simultaneously, spread spectrum has emerged as aleading multiple access technique for these second generation systems.By “spread spectrum” it is meant that each device generates a widebandsignal (e.g. by code division multiple access or by very fast frequencyhopping) which is treated as noise by other devices in the system. See,K. S. Gilhousen, et al., “On the Capacity of a Cellular CDMA System,”IEEE Trans. Veh. Tech., Vol. 4, No. 2, pp. 303-312, May 1991; A. M.Viterbi and A. J. Viterbi, “Erlang Capacity of a Power Controlled CDMASystem,” J. Sel. Areas Comm., Vol. 11, No. 6, pp. 892-900, August 1993.

[0008] In order to fully exploit the advantages of spread spectrumtechniques, however, certain device parameters must be properlyselected, e.g. transmit power levels and cell site selection orassignment. A device may have access to a number of possible cell sitesand a choice as to which cell site it should communicate with must bemade based upon some criterion. Similarly, the transmit power of eachdevice must be determined so as to achieve the desired quality ofservice. One current technique for selecting parameters is described inBlakeney et al., “Mobile Station Assisted Soft Handoff in a CDMACellular Communication System,” U.S. Pat. No. 5,267,261 and in Wheatley,“Transmitter Power Control System,” U.S. Pat. No. 5,267,267, whichdescribe an open loop scheme for power control and cell site assignment.In this scheme, each cell site transmits a pilot signal. The strength ofthe pilot signal is measured at a user's device. The user's device isthen assigned to the cell site whose pilot signal is strongest. Thedevice controls its transmit power level as a function of the receivedpilot signal strength in such a way as to achieve a desired nominalrequired received power at the selected cell site. This solution to thepower and cell site selection problem, however, has several drawbacks.First, the use of a nominal received power level does not allowdifferent users to have different quality of service requirements.Second, the procedure of establishing the desired nominal received powerlevel at each cell is centralized (in that a system controllerdetermines the value of the nominal received power level at a cell) andleaves open the problem of adapting these levels to changing trafficpatterns. Thus, there is a need for an improved method to select deviceoperating parameters in a cellular system, e.g. to select cell sites andto control power levels, to take into account the dynamic nature of thetraffic in the cellular system and the increasingly diverse range ofservices to be carried on wireless networks.

SUMMARY OF THE INVENTION

[0009] In accordance with the present invention, one or more operatingparameters of a communication device in a cellular communications systemare selected based on an interference level measurement at a basestation and on the path gain from the communication device to a cellsite. In preferred embodiments, the operating parameters include theparticular cell site to which the communication device is assigned andthe transmit power level for the communication device. The inventionadvantageously reduces interference levels at cell sites therebyincreasing system capacity while maintaining quality of servicecommitments.

[0010] In a first aspect of the invention, a communication device isassigned to one cell site in a plurality of cell sites in the cellularcommunications system by receiving a measure of the interference levelat each cell site, determining the path gain to each cell cite from thedevice, and assigning the communication device to a particular cell siteas a function of the interference level and the path gain. Additionallyin preferred embodiments, a communication device with an associatedquality of service requirement selects a transmit power level fortransmitting to a selected cell site by receiving a measure of theinterference level at that cell site, determining the path gain to thecell site, and transmitting with a power level determined as a functionof the interference level, required quality of service requirement andthe path gain. The cell site assignment and transmit power levelselections may be periodically updated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Other features and advantages of the invention will becomeapparent from the following detailed description taken together with thedrawings in which:

[0012]FIG. 1 is a diagram of a cellular communications system in whichthe inventive method may be practiced.

[0013]FIG. 2 illustrates a cellular system in which one cell is heavilycongested.

[0014]FIG. 3 illustrates a cellular system in which a heavily congestedcell is contracted.

[0015]FIG. 4 is a flowchart of the inventive method for assigning acommunications device to a cell site as a function of an interferencelevel measurement and a path gain.

[0016]FIG. 5 is a flowchart of the inventive method for selecting atransmit power level for a communications device as a function as afunction of an interference level measurement, a quality of servicerequirement and a path gain.

DETAILED DESCRIPTION

[0017]FIG. 1 illustrates components of a cellular communications systemin which the inventive method may be practiced. Cell 102-k, k=1, 2, . .. , represents a portion of the geographic area 100 served by thesystem. Within each cell is cell site 105-k which is connected to thepublic telephone network. Cell site 105-k establishes a wireless linkover a radio channel with communication device 110 _(k,j), j=1,2, . . ., within cell 102-k for transmitting and receiving information (i.e.data representing text, speech, video, etc.). The wireless link betweenany device 110 _(k,j) and cell site 105-k is comprised of an uplinkU_(k,j) for transmitting information from device 110 _(k,j) to cell site105-k and then to the telephone network and of a downlink D_(k,j) fortransmitting to device 110 _(k,j) information received by cell site105-k via the telephone network.

[0018] The inventive method, described below, selects operatingparameters for a system user's communication device in a cellularcommunications system. In the present illustrative embodiments inparticular, the method selects for the device a cell site and a transmitpower level for that device. In accordance with the invention, theselection is based on an interference level at the cell site and on thepath gain from the device to the cell site. This task is performed foreach and every user in the system in a way that maintains a quality ofservice level for each user. The method is advantageous in that it isdynamic (i.e. able to adapt to changing user configurations and tochanging quality of service requirements of system users) anddecentralized (i.e. each device can determine its own transmit powerlevel and cell site). The inventive method is easier to implement than acentralized scheme, and the method is more robust with respect tochanges in user demands.

[0019] If the cellular communications system utilizes spread spectrumaccess techniques, then several factors make the inventive methodespecially advantageous in selecting a cell site and in selecting atransmit power level. In particular, in spread spectrum systems the C/Iratio, i.e. the ratio of the carrier level to interference level at thecell site, is low because I typically is high (because all other systemusers are transmitting wideband signals that are treated as interferenceby a particular device). Thus, the quality of service in these systemsfor that particular device is very sensitive to changes in I, and theoptimal cell site therefore depends on I as well as on the particulardevice's carrier signal strength C. In contrast, in a narrowband system,I is very low (typically comprising only thermal noise and co-channelinterference) and so the optimal cell site will typically be the cellcite for which C is greatest—making the cell site selection problemtrivial. In any case, I varies too rapidly in the narrowband system tomake adaptive cell site selection and adaptive power control veryeffective. Thus, the inventive method is particularly advantageous forspread spectrum systems because it focuses on the interference level asmeasured at the cell site.

[0020] In a first aspect, the inventive method selects a cell site for adevice. FIG. 2 illustrates a cellular system comprising, illustratively,two cells, 201 and 220, with respective cell sites 205 and 225. Cell 201is heavily congested and operating near capacity, i.e. a large number ofdevices (indicated by the x's within the cell) are simultaneouslyaccessing the system. In contrast, there are few devices (also indicatedby x's) communicating with cell site 225 in cell 220. In this aspect ofthe inventive method, presented in detail below, a device selects a cellsite from among a set of one or more cell sites based on theinterference level at each cell site and on the path gain between thedevice and each cell site. The interference level at a cell site isdefined as the sum of the received power at the cell site from all othertransmitting devices (i.e. not including the power from the device inquestion) plus external noise power (e.g. atmospheric noise) received atthe cell site. The path gain is defined as a scale factor that therequired received power level at the cell site is divided by in order todetermine the required transmit power of the device. As described indetail below, the path gain from the device to the cell site is assumedto be the same as the path gain from the cell site to the device.Advantageously, the path gain from the cell site to the device is easilymeasured, and the measurement is used for the value of the path gainfrom the device to the cell site. The path gain is typically less thanunity and represents, primarily, the degradation in signal strength dueto distance and shadow fading.

[0021] Returning to FIG. 2, the inventive method will typically resultin some devices in heavily congested cell 201 transmitting instead tothe more distant cell site 225 in cell 220, thereby trading a typicallysmaller path gain (due to the increased distance to the cell site) forreduced interference at the cell site 225 (by reason of few devicestransmitting a lower power level in the cell). The method effectivelyreduces or contracts the area served by heavily congested cell sites,and thus the user configuration of FIG. 2 may, using the inventivemethod, be transformed into a user configuration akin to the one shownin FIG. 3. Likewise, if and when cell 220 becomes congested relative toa neighboring cell, cell 220 will contract and the neighboring cell willexpand—in a sense the method allows the cell to “breathe.”

[0022] To be more specific, consider a cellular communications systemwith cell sites k=1,2, . . . , M, at a particular time n. Each user'scommunication device i,i=1,2, . . . N, is allocated to cell site S_(i).Note that the signal received at cell site S_(i) from the j^(th) deviceconstitutes interference at the cell site for the signal received fromthe i^(th) device. Assume each user has a performance level or qualityof service requirement that corresponds to a particular required carrierto interference ratio (C/I)_(i) at the cell site for the signal receivedfrom the i^(th) device. The parameter C is the received signal power ofthe i^(th) device at cell site S_(i) and I is the interference, i.e. thesum of received power of all other signals including thermal noise,measured at the cell site. It will be obvious to those skilled in thatart that the method below can be readily adapted if the interferencelevel I is defined as the received power of all signals, including thesignal from a particular device, received at the cell site. The requiredratio of C/I_(i) is called α_(i). Thus for each user, it is requiredthat$\frac{\left( {{received}\quad {power}\quad {from}\quad i^{th}\quad {device}\quad {at}\quad {cell}\quad {site}\quad S_{i}} \right)}{\begin{matrix}{{\sum\limits_{j \neq i}\quad {{received}\quad {power}\quad {from}\quad j^{th}\quad {device}\quad {at}\quad {cell}\quad {site}\quad S_{i}}} +} \\{{external}\quad {noise}\quad {power}\quad {at}\quad S_{i}}\end{matrix}} \geq \alpha_{i}$

[0023] Consider a particular i^(th) device which, for illustrativepurposes, is assigned to a particular cell site at time n. FIG. 4illustrates steps for selecting a cell site for the i^(th) device at thenext time interval. In step 405 at time n, the i^(th) device receives asignal indicating the amount of interference at each cell site. Forexample, each cell site k may transmit to the i^(th) device a measure ofthe interference level at that cell site. In step 410 the i^(th) devicedetermines the path gain Γ_(i,k) to each cell site k. One simple methodfor computing the path gain Γ_(i,k) to each cell site is to broadcast acontrol signal from each cell site (e.g. a pilot signal). Then if alldevices know a priori the transmitted power level used by each cell sitefor the control signal, the path gain to an individual device from aparticular cell site (which is advantageously assumed to be the same asthe path gain from the individual device to the particular cell site)can readily be determined from the received strength of the controlsignal from that particular cell site at the device. As a result, thei^(th) device can determine the power required to transmit to each cellsite while maintaining the required quality of service in step 415. Thepower P_(i,k) required to transmit from the i^(th) device to each of thek cell sites, k=1,2 , . . . M, is$P_{i,1} = {\alpha_{i}\quad \frac{I_{1}}{\Gamma_{i,1}}}$$P_{i,2} = {\alpha_{i}\frac{I_{2}}{\Gamma_{i,2}}}$$P_{i,M} = {\alpha_{i}\frac{I_{M}}{\Gamma_{i,M}}}$

[0024] The i^(th) device will then select in step 420, at the next pointin time when a new cell site can be chosen, the cell site which requiresthe least amount of power. The device may then optionally transmit atthat power. Of course, α_(i) is common to equations above since α_(i)remains the same for a particular device during time n, and thus thechoice of cell sites is a function only of the interference level andpath gain. In short, selecting the cell site which requires the leastamount of power is the same as selecting the cell site with minimumI_(k)/Γ_(i,k) ratio. As discussed below, the quality of servicerequirement becomes a factor in determining the power level fortransmitting to the selected cell site.

[0025] The above method can be simplified by considering only a subsetof cell sites in the system, instead of all cell sites, when selecting acell site to assign to a particular user. For example, only those m cellsites having the greatest path gain need be considered. Alternatively,only those cell sites having a path gain exceeding a given threshold canbe considered for cell site assignment. The threshold itself can beadaptive to user configurations in the cellular system. Such arestriction in the number of cell sites considered will lead tocomputational savings and reduced complexity in user equipment. Finally,it may be that selecting a new cell site yields little advantage (e.g.only a minimal reduction in transmit power). To prevent unnecessarychanges in cell site assignments and to reduce the rate of change, adamping mechanism can be employed. In this case, the best cell site forthe next time interval is considered only as a candidate cell site. Let$P_{i,{candidate}} = {\alpha_{i}\quad \frac{I_{candidate}}{\Gamma_{i,{candidate}}}}$

[0026] be the power needed to transmit to the candidate cell site, andlet$P_{i,{current}} = {\alpha_{i}\frac{I_{current}}{\Gamma_{i,{current}}}}$

[0027] be the power used to transmit to the currently assigned cellsite. Then the candidate cell site is chosen as the new cell site onlyif P_(i,candidate) is sufficiently less than P_(i,current). For example,a threshold ρ<1 may be chosen so that a change in cell sites occurs onlyif P_(i,candidate)<ρP_(i,current).

[0028] In a second aspect, the inventive method may also be used toselect the transmit power level parameter for a device to communicate toan assigned cell site. Suppose the allocation of cell sites is fixed fora time interval, e.g. the system updates the transmit power levels morefrequently than it updates the cell site assignments. FIG. 5 isflowchart of steps in a method for selecting the transmit power for ani^(th) device with quality of service requirement α_(i) to communicatewith an assigned cell site. In step 505 the device first receives asignal indicating the interference level at the assigned cell site. Thesignal may be broadcast from the assigned cell site, or the signal maybe provided by other means. The path gain Γ_(i) is determined next bythe device as described in step 410 above. In step 515 the power levelfor transmitting from the i^(th) device to the assigned cell site isdetermined according to: $P_{i} = \frac{\alpha_{i}I}{\Gamma_{i}}$

[0029] The theoretical basis for the inventive method is that if the C/Iconstraints of devices in the cellular system can be satisfied, then atany given time there is an optimal assignment of devices to cell sitesin the sense that the power levels (and hence the interference) are atthe minimum levels that achieve the required levels of performance.Iterations of the above method will find this minimal solution. Inpractice, users will move, and the method will track the changes in userconfiguration.

[0030] The above disclosure describes a method of selecting operatingparameters of a device in a cellular system as a function of theinterference level at cell sites in the system and of the path gainsbetween the device and the cell sites. Although the selected operatingparameters discussed above include the particular cell site to which thedevice is assigned and the transmit power level for the communicationdevice, it will be understood by those skilled in that art that otheroperating parameters may be selected.

[0031] The method disclosed herein has been described without referenceto specific hardware or software. Instead, the method has been describedin such a way that those skilled in the art can readily adapt suchhardware or software as may be available or preferable. While the aboveteaching of the present invention has been in terms of power control andcell site selection in a cellular communications system using spreadspectrum access techniques, those skilled in the art will recognize theapplicability of these teachings to other specific contexts. Forexample, the inventive method selects cell sites based only the uplinkbetween users and cell sites. Note that the uplink communication isincoherent in contrast to the coherent downlink—and thus the uplinkrequires a greater C/I ratio for a given quality of service. Thus, anycell site selection procedure that works for the uplink will necessarilywork for the downlink and less transmit power will be required on thedownlink. Similarly, the power control equations can be modified for thedownlink. Further, it has been assumed that each device employs the fullspectrum of available bandwidth. If less than the full spectrum ofavailable bandwidth is used, as for example by using a narrowbandchannel for communicating, the equations above can easily be modified toreflect that the interference level of interest is only the interferencein the narrowband channel.

The invention claimed is:
 1. A method of selecting an operatingparameter for a device in a cellular communication system, the cellularcommunication system comprising one or more cell sites, each cell sitehaving an associated interference level, the method comprising the stepsof: selecting at the device said operating parameter as a function ofthe interference level at each cell site in the cellular communicationssystem and of the path gain between the device and each cell site. 2.The method of claim 1 wherein said operating parameter is a transmitpower level used by the device when the device communicates with a cellsite.
 3. The method of claim 1 wherein said operating parameter is aselection of a particular one of the cell sites said device is to beassigned for communication.
 4. The method of claim 1 wherein theinterference level associated with a particular cell site is a functionof the sum of the received power at the particular cell site from othertransmitting devices.
 5. The method of claim 1 wherein the devicecommunicates with a cell site by transmitting a signal with a transmitpower that is received at the cell cite with a required received powerand wherein the path gain is a scale factor based on the transmit powerof the device and on the required received power at the cell site. 6.The method of claim 1 wherein the function is the ratio of theinterference level to the path gain between the device and each cellsite.
 7. The method of claim 1 wherein the path gain between the deviceand a particular cell site is determined by a method comprising thesteps of: measuring at the device a received signal of known transmittedpower from the particular cell site, and determining the path gain as afunction of the known transmitted power and the power of the receivedsignal.
 8. An apparatus for selecting an operating parameter for adevice in a cellular communication system, the cellular communicationsystem comprising one or more cell sites, each cell site having anassociated interference level, the apparatus comprising: means fortransmitting signals from the device to the cells and for receivingsignals at the device from said cell sites; and means for selecting atthe device said operating parameter as a function of the interferencelevel at each cell site in the [spread spectrum] cellular communicationssystem and of the path gain between the device and each cell site. 9.The apparatus of claim 8 wherein said operating parameter is a transmitpower level used by the device when the device communicates with a cellsite.
 10. The apparatus of claim 8 wherein said operating parameter is aselection of a particular one of the cell sites to which the device isassigned for communication.
 11. The apparatus of claim 8 wherein saidinterference level associated with each cell site is a function of thesum of the received power at each cell site from other transmittingdevices.
 12. The apparatus of claim 8 wherein the device communicateswith a cell site by transmitting a signal with a transmit power that isreceived at the cell cite with a required received power and wherein thepath gain is a scale factor based on the transmit power of the deviceand on the required received power at the cell site.
 13. The apparatusof claim 8 wherein the function is the ratio of the interference levelto the path gain between the device and each cell site.
 14. Theapparatus of claim 8 wherein the path gain between the device and aparticular cell site is determined by an apparatus comprising: means formeasuring at the device a received signal of known transmitted powerfrom the particular cell site, and means for determining the path gainas a function of the known transmitted power and the power of thereceived signal.